The invention relates to an apparatus for the reproduction or recording of data or information with a photodetector and digital error signal and evaluation signal processing which, using an optical scanning device, reads data or information from the data tracks of a recording medium or records data or information in tracks of a recording medium, the procedure being that a light beam is focused onto the recording medium using a focus regulating circuit and is guided along the data tracks using a track regulating circuit and the light beam reflected from the recording medium is reflected onto a photodetector.
In apparatuses for the reproduction or recording of data or information in which, using an optical scanning device, data or information are read from the data tracks of a recording medium or are recorded in tracks of a recording medium, a light or laser beam is focused onto the recording medium using a focus regulating circuit and is guided on the data tracks of the recording medium using a track regulating circuit. The optical scanning device of apparatuses of this type, such as e.g. a CD player, DVD player, a magneto-optical apparatus for the reproduction or recording of data or information, a recording and reproduction apparatus for DRAW discs or a video disc player, has a laser diode, one or more lenses, a prism-type beam splitter, if appropriate a diffraction grating and a photodetector. The structure and function of an optical scanning device, a so-called optical pickup, are described in Electronic Components and Applications, Vol. 6, No. 4, 1984, on pages 209-215.
The light beam emitted by a laser diode is focused onto the recording medium by means of lenses and is reflected from the recording medium onto a photodetector. The data or information stored on the recording medium and the actual values or error signals for the focus and for the track regulating circuit are obtained from the output signals of the photodetector. It is already generally known to use digital servoprocessors for track and focus regulation. Since the error signals for the focus and track regulating circuit have a relatively low frequency in comparison with the data signal, analog-to-digital converters known from audio technology can be used for the analog-to-digital conversion of the error signals. The digital data signal is generated using a pulse shaper which detects the zero crossings of the analog high-frequency signal provided by the photodetector. Furthermore, so-called evaluation signals, such as, for example, the mirror signal and defect signal, are derived from the high-frequency signal provided by the photodetector, which evaluation signals, despite digital servoprocessors, are frequently generated using exclusively analog circuit means, since the digitization of the data or high-frequency signal requires a fast analog-to-digital converter of the kind used for video applications. The types of converter known from digital audio technology cannot be used if only because of the bandwidth of the high-frequency signal detected from a CD by the photodetector. This is cost-intensive and requires a high outlay in apparatuses having a multiple scanning speed, since the entire bandwidth of the high-frequency signal must be digitized. On the other hand, the information derived from the profile of the data signal or of the high-frequency signal has relatively high importance for identifying the cause of an abnormality in the high-frequency signal, which may be, by way of example, a track change or a defect, and is therefore indispensable for reliable functioning of a reproduction or recording apparatus. By way of example, the mirror signal is required in order to be able to distinguish whether the scanning beam is situated on or between the data tracks, since the track error signal is equal to zero in both cases.
Consequently, the advantages of digital evaluation signal processing are offset by the high costs for an analog-to-digital converter for the radio-frequency band or for video applications.
The object of the invention is to provide an apparatus for the reproduction or recording of data or information which requires little outlay despite digital evaluation signal processing and a considerable capability of adaptation to different scanning speeds.
This object is achieved by means of the features specified in independent claims. Advantageous designs and developments are specified in dependent claims.
A first aspect of the invention is that of enabling digital evaluation signal generation with an analog-to-digital converter known from audio technology, even though the data rate of the data or high-frequency signal requires an analog-to-digital converter for the radio-frequency band or for the video signal band.
Analog-to-digital converters for the audio frequency band require a comparatively lower outlay. However, the types of converter known from digital audio technology cannot be used because of the bandwidth of the high-frequency signal detected from a CD by the photodetector even with a single scanning speed. An analog-to-digital converter for the audio frequency band can nevertheless be used to generate a digital evaluation signal by the analog-to-digital converter having fed to it not the high-frequency signal directly but rather at least one envelope of the high-frequency signal. Since the envelope signal has a significantly smaller bandwidth than the high-frequency signal, an analog-to-digital converter known from audio technology can be used for the purpose of digital evaluation signal generation and the advantages of digital evaluation signal processing can nevertheless be utilized. What is involved, then, is an advantageous hybrid solution comprising an analog preamplifier, an analog-to-digital converter for the audio frequency band and a digital processor.
A further aspect of the invention is that of using just one analog-to-digital converter in connection with the digital signal processor of an apparatus for the reproduction or recording of data or information with a photodetector and digital error signal and evaluation signal processing. For this purpose, both error signals and at least one envelope signal of the high-frequency signal detected from the recording medium are fed to an analog-to-digital converter for the video signal band via a multiplexer. In this case, the analog envelope detector preferably has a time constant matched to the lowest high frequency of the data signal detected by the photodetector. The frequency of the data or high-frequency signal detected by the photodetector is, as a rule, higher than that of the information signal since the data signal detected by the photodetector contains additional details concerning location, length or content of the information signal, which are separated out in further signal processing stages. Despite the analog preprocessing of the data signal to form an envelope signal, the variable parameters of one or more evaluation signals, such as, by way of example, of the mirror or defect signal, are adjusted in the digital domain and, as a rule, in an automated manner. This results in a high level of flexibility with regard to the capability of utilizing the apparatus for different recording media and scanning speeds. The invention is based on the principle of using exclusively analog signals of low frequency despite the digital signal processing of all the servosignals including mirror and defect signals, with the result that a narrow-band analog-to-digital converter or an analog-to-digital converter for the video frequency band can be used, all the signals which are necessary for digital error and evaluation signal generation being fed to the said converter via a multiplexer. Digital evaluation or signal processing of the high-frequency data signal in order, by way of example, to be able to distinguish between defects and track change, to adjust the gain in servocontrol loops, to mark incorrect data signals or to count correctly the number of tracks crossed is made possible by virtue of the fact that at least one envelope signal derived from the high-frequency data signal is actually formed prior to the digitization. However, the evaluation of the envelope signal is provided in the digital domain in order to ensure high accuracy and flexibility. Although the data rate, referred to as input frequency here, of a reproduction or recording apparatus for different optical recording media which is detected by the photodetector of the apparatus has a large bandwidth, the control and signal processing are carried out with narrow-band signals. The data or high-frequency signal detected from the recording medium by the photodetector is digitized only in respect of its envelope for the purpose of forming evaluation signals. The envelope signal is formed from the data or high-frequency signal using an analog circuit comprising, by way of example, a rectifier and a capacitor which is charged and discharged in a defined manner. In principle, the proposed apparatus for the reproduction or recording of data, which apparatus requires little outlay despite digital evaluation signal processing, is independent of the way in which the envelope signal is generated.
Nevertheless, two circuit arrangements are specified which can be used for the purpose of envelope signal generation in an advantageous manner.
A first circuit arrangement is proposed, which enables both an upper and a lower envelope of a high-frequency signal to be detected. It is provided particularly when the upper and lower envelope signals cannot be subjected to further processing simultaneously but rather successively. The principle behind the first circuit arrangement for envelope signal generation consists in using the high-frequency signal that is detected to drive an integrator stage via a charge or discharge circuit, which integrator stage is preferably formed by two transistors, a capacitor and a current source, the control transistor of which is connected to a base current compensation circuit. The envelope signal formed by means of the integrator stage is coupled out by means of an emitter follower and simultaneously used as a comparison signal in the charge or discharge circuit. In order to form the upper or lower envelope, the input and the output of the integrated stage are optionally connected to a charge or discharge circuit for the upper or lower envelope. This means, advantageously, that only one integrator stage is required to form the upper and lower envelopes, which integrator stage is used multiply.
In order to provide the upper and lower envelope signals simultaneously, a second circuit arrangement is proposed, which differs from the first circuit arrangement only by the fact that a second integrator stage is provided, with the result that both the upper and the lower envelope signal are constantly available simultaneously. Despite different input frequencies, governed by the type of recording medium and its playback speed, it is possible to use a uniform time constant in connection with the formation of the envelope signal. Deviations from an ideal envelope which is matched exactly to the input frequency can be taken into account during the evaluation of the said envelope in the digital domain. Since all that is involved is the selection of a time constant for the envelope detector and the signal need not be evaluated in the analog domain, it is possible, likewise with little outlay, to provide an envelope detector whose different time constants are set by the digital processor via an IIC bus, by way of example.
The digital signal processing of the error and evaluation signals in a processor connected to the analog-to-digital converter is not discussed specifically at this point since solutions are already known in this respect and the invention is independent of the kind of digital signal processing downstream. All the signals which are necessary for digital error signal and evaluation signal processing are available to a processor after the analog-to-digital conversion of the error and envelope signals.